Visually impaired individuals rely on non-visual liquid level sensing techniques for information on the liquid level in vessels such as cups, pots, measuring cups, coffee and other carafes, jugs, pitchers, and other containers. It may be advantageous at times to determine the static level of a liquid in a vessel, and at times to monitor the level of liquid dynamically, e.g., to follow progress while pouring liquid into the vessel. In the case of a changing level, it is advantageous periodically or continuously to determine the current level of liquid in the vessel, to prevent overflowing and spillage. When seeking to measure out a specific quantity or to determine that the liquid has reached a predetermined level while filling the vessel, it is helpful to obtain an indication immediately.
Certain electronic liquid level sensor/indicators are available to assist the visually handicapped by providing non-visual signals. A measurement based on electrical conductivity may be possible by extending spaced electrodes to a predetermined level and sensing for a drop in resistance when rising liquid reaches the level of the electrodes and closes a circuit including the electrodes. Another electrical property that can be used in a similar way is to sense for a difference in dielectric properties of the material between and adjacent to spaced electrodes. Such properties are distinct for liquid as compared to air.
Capacitive sensors are used in industry to sense the presence of material in bins and other large receptacles. The technique is typically to establish a changing potential difference between capacitor plates, and to detect the difference in charge or current levels involved. Stated another way, a change in capacitance may occur when a material is inserted between conductive plates, in lieu of air, and alters the impedance of a circuit including the plates.
The difference in capacitance that occurs when liquid rather than air is the primary applicable dielectric material can be detected in various ways that are effectively capacitance measurements. The capacitance can affect the charging rate of the plates through a series resistance, or the extent to which higher or lower frequency components are coupled through a tuned circuit, etc. Timing and/or threshold level circuits can be used to determine when a given condition is reached. In order to be useful as an indicator for the visually impaired, an output signal is generated that is perceivable in a non-visual manner, such as audibly or by vibration or the like.
Examples of capacitive liquid level indicating devices are disclosed in U.S. Pat. Nos. 5,406,803 and 6,164,132, for example. Liquid level sensors available to assist the visually impaired in determining changing liquid levels include the “Vibrating Liquid Level Indicator,” the “Sensa Cup Level Indicator MK 111,” and the “Easy Say Stop Liquid Level Indicator,” available on the Web, for example, from Maxi-Aids, Inc., Farmingdale, N.Y. (http://www.maxiaids.com/). These products generally include devices that hang on the rim of a cup, and as so positioned, place spaced electrodes in the volume to be occupied by the liquid, which electrodes become immersed as the cup is filled to their level. A “Talking Liquid Jug” from the same company has a vessel removably carried on a base receptacle that has volume measurement and announcement functions.